Most conventional motor vehicles, such as the modern-day automobile, include a powertrain (sometimes referred to as “drivetrain”) that is generally comprised of an engine that delivers driving power through a multi-speed power transmission to a final drive system, such as a rear differential, axle, and wheels. Automobiles have traditionally been powered solely by a reciprocating-piston type internal combustion engine (ICE) because of its ready availability and relative cost, weight, and efficiency. Such engines include 4-stroke compression-ignited diesel engines and 4-stroke spark-ignited gasoline engines.
Hybrid vehicles, on the other hand, utilize alternative power sources to propel the vehicle, minimizing reliance on the engine for power, thereby increasing overall fuel economy. A hybrid electric vehicle (HEV), for example, incorporates both electric energy and chemical energy, and converts the same into mechanical power to propel the vehicle and power the vehicle systems. The HEV generally employs one or more electric machines that operate individually or in concert with an internal combustion engine to propel the vehicle. Since hybrid vehicles can derive their power from sources other than the engine, engines in hybrid vehicles can be turned off while the vehicle is propelled by the alternative power source(s).
Series hybrid architectures, sometimes referred to as Range-Extended Electric Vehicles (REEVs), are generally characterized by an internal combustion engine in driving communication with an electric generator. The electric generator, in turn, provides power to one or more electric motors that operate to rotate the final drive members. In other words, there is no direct mechanical connection between the engine and the drive members in a series hybrid powertrain. The lack of a mechanical link between the engine and wheels allows the engine to be run at a constant and efficient rate, even as vehicle speed changes—closer to the theoretical limit of 37%, rather than the normal average of 20%. The electric generator may also operate in a motoring mode to provide a starting function to the internal combustion engine. This system may also allow the electric motor(s) to recover energy from slowing the vehicle and storing it in the battery by regenerative braking.
Parallel hybrid architectures are generally characterized by an internal combustion engine and one or more electric motor/generator assemblies, each of which has a direct mechanical coupling to the power transmission. Most parallel hybrid designs combine a large electrical generator and a motor into one unit, providing tractive power and replacing both the conventional starter motor and the alternator. One such parallel hybrid powertrain architecture comprises a two-mode, compound-split, electro-mechanical transmission which utilizes an input member for receiving power from the I/C engine, and an output member for delivering power from the transmission to the driveshaft. First and second motor/generators operate to rotate the transmission output shaft. The motor/generators are electrically connected to an energy storage device for interchanging electrical power between the storage device and the first and second motor/generators. A control unit is provided for regulating the electrical power interchange between the energy storage device and motor/generators, as well as the electrical power interchange between the first and second motor/generators.
Transitions in operating states of hybrid powertrain systems can produce clunks (i.e., audible noises) and jerks (e.g., physical lurches) as slack, resulting from driveline lash in the gear train, is taken out of the driveline, and driveline components impact one another. “Driveline lash” refers to the play or slack in the rotational position of the driveline components, such as clearance between transmission splines, interleafed gearing teeth, etc. When the engine transitions from exerting a positive torque to exerting a negative torque (or being driven), the gears in the transmission or driveline separate at the zero torque transition point. Then, after passing through the zero torque point, the gears again make contact to transfer torque. Such clearance is generally necessary to accommodate build variation and thermal expansion of powertrain components.
Gear lash, clunks, and jerks have the potential to occur during vehicle operations including: when the operator changes transmission gears, e.g. from neutral/park to drive or reverse; when the operator tips into or out of the throttle; or when the vehicle is operated on an inclined surface. Lash action occurs, for example, as follows: torque-generative devices of the powertrain exert a positive torque onto the transmission input gears to drive the vehicle through the driveline. During a subsequent deceleration, torque input to the powertrain and driveline decreases, and gears in the transmission and driveline separate. After passing through a zero-torque point, the gears reconnect to transfer torque, in the form of motor braking, electrical generation, and others. The reconnection of the gears to transfer torque can result in gear-to-gear impacts, with resulting clunks and jerks, which may be perceptible to the vehicle operator, and can negatively affect powertrain and transmission durability.